We report on a degradation mechanism in thin-film photovoltaic (PV) modules activated by damp heat and voltages similar in magnitude to those generated by PV modules in power generation systems. This mechanism, which appears to be an electrochemical process involving the soda-lime glass superstrate with its conductive SnO2:F layer, can be greatly accelerated by subjecting modules to elevated temperatures and humidity, both of which increase the leakage currents between the frame and the active PV layers. Water vapor can affect the module damage in two ways: (1) by enhancing leakage currents, and (2) by entering through the module edges, it appears to promote the chemical reaction responsible for the SnO2 corrosion. Damage has been found to occur in both a-Si and CdTe modules. (C) 2002 Elsevier Science B.V. All rights reserved.